1. Field of the Invention
The present invention relates to the use and manufacture of proton exchange membranes and their application in membrane and electrode assemblies for fuel cells, particularly fuel cells used for the direct oxidation of organic fuels such as methanol, ethanol, dimethoxymethane, and trimethoxymethane.
2. Background of the Related Art
Fuel cells comprising proton exchange membranes are the focus of increasing research efforts in the development of new and clean power sources. These efforts have shown that polymer electrolyte membranes (PEM) also referred to as proton exchange membranes, offer a number of advantages over conventional electrolytes when used in electrochemical devices such as fuel cells and water electrolyzers. Unfortunately, these electrolytes must remain hydrated to retain ionic conductivity, which limits their maximum operating temperature to 100.degree. C. at atmospheric pressure.
This disadvantage of known PEM materials is particularly highlighted in systems in which a polymer electrolyte with high conductivity at temperatures in excess of 100.degree. C. would be useful. One such application is the H.sub.2 /O.sub.2 fuel cell that utilizes reformed hydrogen from organic fuels (methane, methanol, etc.)
One alternative to the use of pure hydrogen or enriched hydrogen derived from the reformation or partial oxidation of organic fuels in PEM fuel cells which circumvents the aforementioned disadvantage is to oxidize the fuel directly in the fuel cell rather than to employ an intermediate conversion step to hydrogen. Methanol is particularly attractive in this respect since it possesses a high energy density and, because it is a liquid at ambient temperatures like gasoline, much of the infrastructure is already in place for its safe storage and handling.
Methanol fuel cell systems currently under development use low-temperature steam reformers in conjunction with fuel cell stacks to generate power from methanol in indirect systems. By "indirect" it is meant that methanol fuel is processed (by a reformer) before it is introduced into the fuel cell stack. However, the system can be vastly simplified, and the overall system thermal efficiency can be improved if direct anodic oxidation of methanol is achieved at low polarization. A direct methanol fuel cell will also be preferred for vehicular applications because its weight, volume, start-up and load-following characteristics should be more attractive than the more complex indirect systems.
A direct methanol fuel cell (DMFC) which utilizes a proton-exchange membrane as the electrolyte, has the capability to replace batteries in small, portable applications. Analyses indicate that the performance level of this fuel cell at the present time is almost high enough that such a small version of a direct methanol proton exchange membrane fuel system (DMPEMFC) could be competitive with the highest energy density batteries available in terms of size and weight.
The direct methanol fuel cell is a potentially attractive power source for vehicles and other applications in the military as well as the commercial sectors. Benefits to be derived from the use of direct methanol fuel cells as power sources include dramatic reductions in emissions of air pollutants, reduction in the nation's dependence on imported petroleum since methanol can be made from indigenous fuels such as coal and natural gas and also from renewable sources such as wood and biomass, and an overall increase in vehicle energy efficiency. Use of liquid methanol fuel avoids the difficulties and hazards associated with the handling of gaseous reactants such as hydrogen. Vehicles powered by DMFCs have the potential for a very large market in California, the New England States, and other states in the Northeast that have mandated the introduction of zero-emission vehicles by the end of the decade.
Several different types of fuel cells have been evaluated for direct methanol operation, including molten carbonate fuel cells, aqueous carbonate fuel cells, sulfuric acid fuel cells, and phosphoric acid fuel cells. However, due to high projected power densities, low operating temperature and pressure, and the potential for system simplification, the fuel cell system receiving the most attention for transportation applications, using methanol as a fuel, is the proton exchange membrane fuel cell (PEMFC). This fuel cell uses a hydrated sheet of perfluorinated ion exchange membrane as a solid electrolyte in the fuel cell; catalytic electrodes are intimately bonded to each side of the membrane. Membranes of this type are sold commercially, for example, under the trademark NAFION.RTM. from E.I. du Pont de Nemours and Company.
However, a major disadvantage limiting the use of known PEM methanol-air fuel cells is that currently available PEM electrolytes do not totally exclude methanol. The PEM-based fuel cell is characterized by the use of a polymer membrane, typically a polyperfluorosulfonic acid such as NAFION.RTM., as the electrolyte. These types of hydrophilic membranes are extremely permeable to both water and methanol, particularly at elevated temperature. In the PEMFC, methanol dissolved in water is supplied to the anode as a liquid, and, as a result of the high permeability and the absence of methanol at the cathode, methanol crosses over from anode to cathode via diffusion. Methanol permeates from the anode chamber of the PEMFC across the membrane, adsorbs on the cathode catalyst, and is oxidized, resulting in a parasitic loss of methanol fuel and reduced fuel cell voltage. Performance losses of 70-150 mV at a given current density have been observed at the cathode of PEMFCs with a direct methanol feed. High rates of methanol crossover from the anode to the cathode may ultimately lead to depolarization of the cathode resulting from oxidation of the fuel at the cathode.
Another subtle yet critically important consequence of the extreme volumes of water and methanol that reach the cathode influences the structure of the cathode, the cathode flow field, and hence, DMFC stack design. Recent experiments with methanol-air stacks have shown that excessively high oxidant flow rates, over five (5) times the stoichiometric requirements, are required to remove excess liquid from the cathode to prevent flooding and maintain stable stack operation. For stacks with high pitch (number of cells per inch), large air pumps are required because of the significant pressure drop within the stack hardware, and the necessary use of larger pumps increases undesirably the level of parasitic electrical power. A means of controlling or removing at least some of the fluids transported through a proton exchange membrane from anode to cathode in an electrochemical cell before they arrive at the cathode is highly desirable.
One avenue of investigation for the reduction of methanol crossover in direct methanol fuel cells involves the modification of known ionomeric polymer systems as a means of enhancing their resistance to methanol crossover. Examples of such modified membranes include membranes with inorganic fillers and multi-layer membranes where the layers have properties that differ from one another. For example, a membrane based on polystyrene sulfonic acid, crosslinked with polyvinylidene fluoride (KYNAR) has been proposed. While this membrane allowed a reduction in methanol crossover it also reduced achievable current density levels.
Attempts have also been made to devise new multi-layer laminates which include proton permeable, methanol impermeable layers within the membrane structure. As an example, a three layer membrane, the middle layer of which was a thin palladium hydride, has been fabricated. The laminate was mounted subsequently in a hydrogen-oxygen fuel cell and subjected to methanol-saturated hydrogen to evaluate the resistance to methanol crossover. Experiments demonstrated that fuel cell performance degradation due to methanol crossover in this hydrogen fuel cell was eliminated. However, it was later determined that the membrane did not work in the DMFC.